1. Field of the Invention
The present invention relates to a high-frequency choke circuit and, more specifically, to a high-frequency choke circuit for preventing the passage of high-frequency waves such as microwaves and millimeter waves to ensure isolation between circuits.
2. Prior Art
In microwave and millimeter wave circuits, a high-frequency choke circuit is indispensable for supplying a DC bias to semiconductor devices, for instance. The high-frequency choke circuit is generally comprised of a high-impedance section and a low-impedance section (capacitance section). The capacitance section is an important factor in miniaturization of the entire circuit, in particular, because it requires an increasingly wider area with decreasing frequency.
Various circuit configurations have been proposed to advance miniaturization. The high-frequency choke circuit disclosed in Japanese Patent Laid-open Publication No. Hei. 4-284002 will be described below as an example of such configurations.
FIG. 1 is a schematic sectional view showing a configuration of the conventional high-frequency choke circuit in the above publication. This high-frequency choke circuit is formed in a multilayered substrate. More specifically, a high-impedance line 51 and a first grounding conductor 52 are formed on surface layer P1, a low-impedance line (capacitance land) 53 is formed on second layer P2, and a second grounding conductor 54 is formed on third layer P3. The high-impedance line 51 and the capacitance land 53 are connected in series via a through-hole 55. The capacitance land 53 is interposed between the grounding conductors 52 and 54.
In the above conventional high-frequency choke circuit, input and output lines that are connected to a device such as a GaAs FET and interconnections of a matching circuit, etc., are also formed on surface layer P1. Electromagnetic coupling is thus likely to occur through space, which prevents the choke circuit from having sufficient high-frequency interruption and shielding effects. As a result, an active circuit such as an amplifier circuit cannot operate stably enough.